Enter the text here that is the new abstract information for your application. This section must be no longer than 30 lines of text. Given the various tools that were developed to study Glycosaminoglycan (GAG) mixtures (specific aims 1), studying glycan mixtures in the context of glycans present in the upper airways became an obvious extension of this work. The glycan mixtures - especially the sialylated glycans present in the epithelial cells- provided the important reference point to these being critical receptors for Influenza A viruses. We had furthermore established a foundation to study the protein-GAG interactions, using various GAG degrading enzymes over the years. Based on the fact that many of these enzymes, and other GAG or glycan binding proteins, have poor sequence identity [this includes key sulfatases, fibroblast growth factors or influenza virus hemagglutinin (HA)], we developed newer tools to study GAG-protein interactions, which were broadly applicable to protein-glycan interactions. Newer analytical and structural tools were developed under specific aims 1 and as part of specific aims 2. We developed not only GAG/glycan databases, but also protein structural databases to study the diversity of glycan- protein interactions. Integration of these two approaches-glycan conformation coupled with protein structure and database-enabled us to tackle a fundamental question related to glycan specificity of influenza A viruses, which turned out to be the ideal model system to validate our approach. Given extensive glycan diversity and the mixture of glycans present in the apical epithelium, for the first time we demonstrated that glycan topology is a key determinant for human adaptation of influenza viruses (published in Nature Biotechnology). Our prediction that the then emerging 2009 H1N1 swine flu virus was not efficient in transmission (published in Science 2009) was further validated and observed due to poor transmission of this virus in humans (published in NEJM 2009). More recently, with the outbreak of the 2013 H7N9, we rapidly investigated the receptor specificity of the H7N9 HA and published our observations (along with our predictions on H5N1) back-to-back in the journal Cell. The R37 grant application will address key questions related to glycan topology and HA binding using complementary analytical and structural approaches.